Aquaponics, the science bit

Last update: 10.07.09 First posted: 14.06.09 by Nick in Blogs

New fish arriving from Riggs, 12 of 3-4” Koi Carp

The fish look very happy and the leafy salad vegitables look good but the more nutrient hungry tomato plants are lanky indicating that thier demands are not being met.

I discussed getting plant nutrient levels in the water analysed with Graham Proudlove our local fishy academic. Just recently,however I spotted a range of indicator strips and test kits in the very well stocked fish department of Riggs garden centre.
Although these test strips are primarily aimed at fish health I thought they might give some clues as to the state of play for the plants.

I bought this one

You dip the strip (on the right of photo) in the tank,wait a minte and compare colours on the scale on the tin.

Starting at the bottom of the strip, the yellow square shows a pH of 6.5, a bit low, but ok for carp and in a way its a good sign, ammonia from the fish would tend to raise the pH.an indication that the purification system is working.

The next up is KH,bicarbonate hardness.The fish are in rainwater which is naturally soft, low mineral content. The KH of my tank water is pretty low,not a bad thing, but KH acts as a Ph buffer, resisting rapid changes and stabilising the system, so maybe it needs some tweeking.

The 3 squares next up, are a measure of GH, general hardness, thats the level of calcium and magnesium salts. I added some oyster shell, a calcium supliment borrowed from the poultry department at Grumpy’s farm. The squares are now,green, green, purple indicating the water is in the 6 degree level,which is good. It had been lower so the oyster shell has helped.
Reading up about water hardness indicates that calcium carbonate,the main ingredient of oyster shells will increase both KH and GH,so I will slowly add more.

The next squares going upward,the top two, are the most interesting,and the awnser to my “lank Vegitables” problem. The top square indicates nitrates (plant food) levels of next to nothing. The nitrite indicator square is equaly no show.

Its good news and bad news realy.The good news is that theres absolutley no build up of fish poo,the friendly bugs and plants are sucking it all up.The bad news is that theres nowhere near enough nutrients for the plants,explainig the lank growth. lank tomato plant

There are however other factors. all this measuring is about fish health,not aimed at plant nutrition. The other major plant nutrients,phosphate and potasium have not been measured. Chatting to Ryan at the aquarium shop in Riggs garden centre, he suggested using some tap water, apparently its very high in phosphates locally, to the extent that it promotes heavy algal growth in ponds. Tomatoes like phosphate so its probably a top tip! Ok, so ive added 5 litres of tap water to the 100L tank,not too much at a time, fish dont like the chlorine,I will add more in a week.
But this does not adress the nitrogen issue.

not to worry,aliens ain’t invaded,the green thing is a water hiacynth!
Its a bit of a carefull balance of fish vs growing plants, and a balance that changes as temperature, light levels etc etc vary. It seems the options are to reduce the plants or increase the fish, so I went back to Riggs to buy the 12 fish in the top picture.
We now have about 20 3-4” fish in an 100 litre tank. My plan is to keep adding fish at weekly intervals untill the water nitrate levels begin to show a rise.
More anon!